Automotive design and manufacture present a unique set of problems in the selection of materials and processes used in the manufacture of automobile bodies. As a form of transportation, automobiles are unique because most buyers want a vehicle to have a certain individual styling. A recent trend in the automobile industry is toward production of distinctive vehicles styled to attract specific groups of consumers. This change has required the car builder to shift production from a few models manufactured in large volumes to a larger number of more distinctive body styles. These developments have demanded from the manufacturer both styling flexibility and reasonable tooling costs for each body style.
For many years, car body members and panels have been made predominantly from sheet metal. However, car builders now generally recognize that future use of plastics for exterior car body members and panels, rather than sheet metal, may provide a solution to meeting the demand for more unique body styling and reduced tooling costs. With the increasing concern for weight-reduction, car builders have also turned their attention to the use of plastics as an alternative to heavier metal parts. For instance, certain exterior car body parts of many automobiles now on the road are made from lighter-weight plastic. These parts include bumpers, rocker panels, fender extensions, window and door moldings, and the like.
As reported in Plastics World, November 1986, p. 30 et seq., a number of advanced corporate development programs now in process are also seeking solutions to the problem of commercially manufacturing from plastics the larger exterior car body panels such as hoods, roofs, deck lids, and in fact, the entire car body, if possible. The concept of making a production car body from a material other than sheet metal dates back to at least as early as the mid-1950's, when the Chevrolet Corvette was first manufactured with a fiberglass body. Developments in plastic resin technology in recent years have resulted in more sophisticated plastic materials of higher impact strength than fiberglass. Polycarbonates are an example. These developments in plastics technology have caused many plastics manufacturers, for about the last decade or so, to seek solutions to the problem of commercially producing the entire car body at a reasonable cost from these later-developed high-strength plastic materials. Recent development efforts have also been directed toward producing plastic car bodies from various alternative plastics molding processes, including use of SMC (sheet molding compounds) and RIM (reaction injection molding) techniques.
Development of a production process for making exterior car body members and panels from plastics requires solutions to a number of technical problems. These parts must be manufactured at reasonable costs for tooling, assembly and materials. The end product also must meet certain quality requirements. For instance, the resulting car body panel must have structural capabilities such as impact strength and resistance to mechanical stress necessary to compete with sheet metal. It must also have a paint coat with a defect-free and durable exterior surface. An automotive quality paint coat must meet certain specifications for a large number of physical properties in order to be capable of use as an exterior automotive paint coat. These properties include gloss; distinctiveness-of-image; hardness; abrasion resistance; weatherability such as UV resistance; impact strength; thermal stability, namely, resistance to extreme high and low temperatures; gasoline and acid resistance; cleanability; adhesion to the underlying car body panel; resistance to water and humidity exposure; and hiding ability or opacity of the paint coat.
In the past, a conventional production process for applying an exterior paint coat to car bodies made of sheet metal has involved transporting the preformed auto bodies to application lines in the plant where the car bodies are dipped in paint, followed by transporting them to a separate location for baking the paint coat and waiting until the hardened paint coat dries thoroughly. Most paint systems today are acrylic enamels which are cross-linked into a hard, glossy, durable paint coat during baking. Following painting, the car bodies are transported back to the production plant for further assembly operations. The painting of plastic car body members has typically involved manually spray painting the plastic parts in a separate painting facility, followed by drying, and then transporting the finished parts back to the assembly operation. Conventional painting of sheet metal car bodies and plastic parts is expensive and is a significant factory problem with respect to environmental protection, workers' safety, corrosion treatment, and waste disposal. It is estimated that about one third of the total capital investment in an automobile production facility today is involved in painting car body members and panels.
In recent years, metalized laminating techniques have been used for forming a reflective, weather-resistant metal surface on molded plastic automotive trim parts. These techniques have not yielded plastic trim parts with a paint-coated surface capable of exterior automotive use. Such plastic trim parts have experienced difficulties in maintaining reflectivity and avoiding surface defects under mechanical impact and environmental exposure.
New car body applications of molded plastic materials can develop if the manufacturer can find a way to commercially produce plastic car body panels with a paint coat having the durability and appearance properties necessary for exterior automotive use. Moreover, if in-mold painting or coating processes can be developed as an alternative to conventional auto painting, then auto assembly plants can be more compact, and capital costs and environmental and safety problems associated with conventional car painting at the factory site can be avoided.
Many corporate product development efforts have sought alternatives to conventional painting of molded plastic exterior car body panels and parts, but without any known success to date. A number of problems must be overcome to develop an economical production-type paint system and process for applying a paint coat capable of exterior automotive use for molded plastic exterior car body panels and parts so as to eliminate conventional spray painting operations. For instance, use of the cross-linked acrylic enamel paint systems which are commonly baked on the sheet metal car bodies to produce a tough, glossy finish cannot be used in painting plastic car bodies because of temperature limitations. One approach, which is the subject of this invention, involves developing a paint-coated laminate which can be used to replace the conventional painted finish on the exterior of an automobile body. The laminate is made from a paint coat applied to a casting sheet by paint coating techniques. The dried paint coat is then transferred from the casting sheet to a laminate panel by dry paint transfer techniques. The coating operation permits use of high temperature resistant paint systems capable of producing a tough, glossy finish. The laminate is later thermoformed into a complex three-dimensional shape and then bonded or integrally molded to an underlying plastic car body member or panel. Injection-cladding techniques can be used to manufacture a molded plastic part and simultaneously bond the laminate to the exterior of the molded plastic part.
A number of technical problems must be overcome in order to use such a laminate in a thermoforming and injection-cladding process, while maintaining a defect-free painted surface with high gloss and durability properties throughout the thermoforming and injection-cladding steps. For instance, the laminate must be heat and vacuum-formable into a complex three-dimensional shape without cracking, deglossing, stress whitening or creating other surface defects. A paint coat on such a laminate can require a substantial amount of pigment in order to provide the necessary color density or opacity and distinctiveness-of-image. However, it has been found that use of pigments in a paint coat can cause deglossing of the surface when a paint-coated laminate is thermoformed. Deglossing may even occur in a nonpigmented clear coat during thermoforming.
In addition to surface gloss requirements, the finished paint coat must be defect-free. Defects must not be produced by the thermoforming step, and the laminate also must be bonded or molded to the underlying plastic substrate in a manner that hides any defects that may be present in the substrate material.
Moreover, a finished paint coat may have a reasonably high surface gloss, but still not have the desired visual appearance known as distinctiveness-of-image. This property relates to the mirror-like character of the finish and is measured by the clarity of an image reflected by the finished surface. It is difficult, in a thermoforming operation, to produce an exterior automotive paint coat with a high gloss level and a high distinctiveness-of-image.
Durability properties are also critical in producing a paint coat capable of exterior automotive use. The paint coat must avoid exhibiting defects when exposed to mechanical impact and avoid deterioration of the surface from exposure to chemicals and to the weather.
A paint system that produces the toughness or hardness necessary for exterior automotive use also must have the elongation properties and heat resistance necessary to allow thermoforming of the paint coat into complex three-dimensional shapes without cracking, deglossing, producing stress lines or other surface non-uniformities, or otherwise degrading the finish. Large pigment levels also adversely affect the strength and alter elongation properties of the paint coat. In addition, reliable bonding of the paint coat to the laminate and bonding of the laminate to the underlying substrate material are essential.
Thus, the desired paint system should have a critical combination of many physical properties in order to produce a surface capable of exterior automotive use, while retaining desired surface characteristics after the laminating, thermoforming and injection-cladding or molding steps have been carried out. However, some physical properties tend to be mutually incompatible in such a process. For instance, a paint system may have good qualities of durability such as hardness, toughness, weatherability and the like; but the same paint system may not have sufficient elongation to be heat-formed into a complex shape without cracking or otherwise losing its durability properties. Other paint systems may degloss when heat-formed into a complex shape. Some paint systems have sufficient elongation to permit thermoforming into the desired complex shape, but they are too soft and therefore lacking in the necessary hardness or durability properties.
In summary, there is a need for an economical production process for manufacturing highly contoured molded plastic exterior car body members or panels with a laminated paint coat having both the durability and the gloss and other appearance properties sufficient for exterior automotive use. Laminating techniques for applying a paint coat to such a molded plastic part can provide a valuable alternative to conventional painting of exterior car body members. Certain properties, such as glass-smooth, defect-free surfaces and uniform paint coats, are better produced by laminating techniques than by conventional painting. Capital costs also can be reduced and environmental problems can be alleviated. Laminating techniques require, in addition, however, a paint system and processing techniques capable of producing and maintaining, throughout processing, the durability and elongation properties, opacity, gloss and distinctiveness-of-image levels, and defect-free surface necessary for exterior automotive applications. The present invention solves these problems.